1. Field of the Invention
This invention relates to a flexible printed wiring board on which various surface mounting type electronic components are mounted, and which is built into an electronic device such as a mobile phone and an optical disc drive, and a manufacturing method thereof.
2. Description of the Related Art
In recent years, as a component which configures wiring of electronic circuits in various electronic devices, a flexible printed wiring board having flexibility has been used. As a manufacturing method of a commonly used flexible printed wiring board, firstly, manufactured is such a copper-clad lamination board for use in a flexible printed wiring board (hereinafter, referred to as FPC copper-clad lamination board) that a copper foil is glued to one surface or both surfaces of a base film which is composed of a polyimide film etc., by use of a bonding material etc., and the copper foil of the FPC copper-clad lamination board is selectively etched, to form a conductor pattern. Next, a surface of the conductor pattern is coated with an insulation protective material having an opening portion for use in component mounting, and thereafter, surface processing such as plating processing and solder paste printing is applied to a surface of the conductor pattern which is exposed from the opening portion for component mounting, and finally, an outer shape forming process is carried out by use of a metal mold etc., to manufacture a flexible printed wiring board with a predetermined shape.
Here, as a method of forming an insulation protective material on a surface of a conductor pattern, two methods are normally used. A first method is a method of pasting an insulative cover layer film by an bonding material, and a second method is a method of pattern-printing cover coat ink by a screen printing method etc.
The first method is of a so-called cover lay type, and it is common to use a polyimide film having an adhesion layer on a surface of a film which becomes a base, and since it excels in endurance, it is used for a flexible printed wiring board which requires flexibility. Meanwhile, a punching process is preliminarily applied to the cover lay film by use of a metal mold etc., to form an opening portion for component mounting, and alignment with a conductor pattern is carried out and then, thermocompression bonding is carried out, and thereby, an insulation protective material is formed on a surface of the conductor pattern.
The second method is of a so-called cover coat type, and is used mainly in tape auto bonding (TAB), and enables manufacturing with roll-to-roll, and excels in productivity. As cover coat ink, heat-hardening ink, which is produced based on epoxy type resin, is typically used, and therefore, it lacks of endurance as compared to the first method in which the cover lay film is used, and it is not adopted so many in a flexible printed wiring board which requires endurance. In addition, cover coat ink, which is produced based on polyimide type modified resin having flexibility as well as heat-resistance, has been developed, but it does not come up to endurance and mechanical strength of the cover lay film, and there is no actual result of adoption in an application which requires endurance.
In the meantime, in the such like flexible printed wiring board, in recent years, in electronic devices such as mobile phones and optical disc drives, high-density mounting, going with endurance against bending and realization of fine wiring, has been demanded. However, in the above-described first method, the component mounting opening portion in the cover lay film having endurance is formed by a punching process through the use of a metal mold, and therefore, if fine opening portions for component mounting, which are concentrated in one location, are formed, dimensional and positional accuracy is not obtained. In addition, a plurality of expensive metal molds become necessary, which is of high cost, and complicated processes become necessary and therefore productivity is lacking. Further, in the above-described second method, sufficient endurance could not be obtained.
Consequently, in order to realize both of endurance against bending and high-density mounting, such a method that a cover lay film is pasted in a portion requiring endurance of a flexible printed wiring board, e.g., a bending portion which is bent at the time of incorporating in an electronic device, and thereafter, photosensitive resist, which is composed of acrylic type resin etc., is formed on a portion where high-density mounting is carried out, e.g., a component mounting portion, and light exposure and development are applied thereto in a predetermined pattern, and thereby, an insulation protective material having opening portions for component mounting is formed, is disclosed, for example, in JP-A-10-4256. In a flexible printed wiring board of JP-A-10-4256, a bending portion is insulation-protected by a paste type cover lay film, and a component mounting portion is insulation-protected by a photosensitive cover lay film, and therefore, it can realize both of endurance and high-density mounting.
However, the above-described related art has problems as follows.
(1) In a related flexible printed wiring board using a cover lay film and photosensitive resist, tremendous man-hours for resist application or coating, light exposure, development and hot curing are necessary, in order to form the photosensitive resist, and productivity is lacking. Particularly, in the flexible printed wiring board of JP-A-10-4256, as a photosensitive cover lay film, one, which is formed in an individuated sheet form, is used and it is pasted by a batch process, and therefore, in a process of pasting a cover lay film and later processes, it is common that each wiring board is processed in an individuated sheet form, and a forming process of photosensitive resist as described above becomes extremely complicated and troublesome cork. Therefore, it has such a problem that productivity is materially lacking.
(2) In addition, in a portion where a paste type cover lay film and photosensitive resist (photosensitive cover lay film) are overlapped, there is such a necessity that the photosensitive resist completely fill a step of the cover lay film and closely overlaps strongly. But when the photosensitive resist is formed from above of the cover lay film, it has such a problem that a filling characteristic of the photosensitive resist is insufficient, and adhesive strength to the cover lay film is not strong, and protection performance of a conductor pattern is lowered and endurance is lacking.